Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329, which is incorporated herein by reference in its entirety. The carbonylation catalyst contains a Group VIII metal catalyst, such as rhodium, which is either dissolved or otherwise dispersed in a liquid reaction medium or supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. Most typically and usefully, the reaction is conducted by continuously bubbling carbon monoxide gas through a liquid reaction medium in which the catalyst is dissolved. The production of acetic acid by the carbonylation of methanol requires the use of large amounts of iodide compounds in the reaction medium. The composition of the reaction medium is a key aspect for efficiently producing acetic acid, stabilizing the rhodium catalyst, and reducing byproduct formation. In particular, the formation of byproducts such as hydrogen iodide that promote water gas shift reactions and increase corrosion should be reduced.
Haynes (2010) “Catalystic Methanol Carbonylation,” reports that raising methyl acetate concentration results in lower hydrogen iodide concentration, which tends to inhibit oxidation of [Rh(CO)2I2]− to give [Rh(CO2)2I4]−, thus suppressing the water gas shift reaction significantly.
U.S. Pub. No. 2013/0310603 discloses an acetic acid production process comprising a reaction step for continuously allowing methanol to react with carbon monoxide in the presence of a catalyst system comprising a metal catalyst (e.g., a rhodium catalyst), an ionic iodide (e.g., lithium iodide), and methyl iodide in a carbonylation reactor; and in the process, (i) the concentration of the metal catalyst is maintained at not less than 860 ppm on the basis of weight, the concentration of water is maintained at 0.8 to 15% by weight, the concentration of methyl iodide is maintained at not more than 13.9% by weight, and the concentration of methyl acetate is maintained at not less than 0.1% by weight, in a whole liquid phase in the reactor, and/or (ii) the concentration of the metal catalyst is maintained at not less than 660 ppm on the basis of weight, the concentration of water is maintained at 0.8 to 3.9% by weight, the concentration of the ionic iodide is maintained at not more than 13% by weight, the concentration of methyl iodide is maintained at not more than 13.9% by weight, and the concentration of methyl acetate is maintained at not less than 0.1% by weight, in a whole liquid phase in the reactor. Many others such as U.S. Pat. Nos. 4,994,608; 6,211,405; 6,657,078; 7,005,541; 7,476,761; and 7,884,241 have described overlapping component ranges in the reaction medium. U.S. Pat. App. Pub. No. 2013/0310603 claims that a reaction medium having these concentrations inhibits an increase in concentration of hydrogen iodide in the carbonylation reactor.
U.S. Pat. No. 4,733,006 discloses a process wherein an olefin, an alcohol, or an ester, halide or ether derivative of said alcohol is reacted with carbon monoxide in a liquid phase in the presence of a catalyst system that contains (a) a rhodium component, and (b) an iodide or bromine component. By passing at least a portion of the liquid reaction mass from the reaction zone to a separation zone of substantially lower CO partial pressure, at least a portion of the carbonylation products, as well as unreacted carbon monoxide, inert gases and unreacted olefin, alcohol, or alcohol derivatives are vaporized and can be withdrawn from the separation zone. Precipitation of the rhodium catalyst under carbon monoxide deficient conditions is prevented or retarded by addition to the carbon monoxide deficient zones of the system of a catalyst stabilizer which is a germanium (IV) compound, an alkali metal compound, and mixtures thereof. The alkali metal compound may include lithium acetate.
U.S. Pat. No. 5,001,259 discloses methanol that is reacted with carbon monoxide in a liquid reaction medium containing a rhodium catalyst stabilized with an iodide salt, especially lithium iodide, along with alkyl iodide such as methyl iodide and alkyl acetate such as methyl acetate in specified proportions. U.S. Pat. No. 5,001,259 deals with the equilibrium existing in the reaction medium between lithium iodide and lithium acetate: LiI+MeOAcLiOAc+MeI. With decreasing water content the lithium acetate content of the reaction medium increases, this effect being greater when 12 wt. % methyl acetate is present as compared with 4 wt. %. U.S. Pat. No. 5,001,259 reports that this equilibration between lithium iodide and lithium acetate which is dependent on the water concentration of the reaction medium has been found, incidentally, to have no adverse effect on the behavior of the catalyst system. This equilibrium will allow the increasing of the lithium iodide concentration of the reaction medium by adding, if desired, lithium acetate or other lithium salts. Because of this equilibrium, one cannot distinguish the effect of lithium iodide from that of lithium acetate on the reaction rate, and it is possible that both the lithium iodide and lithium acetate increase the reaction rate, especially with catalyst solutions with low water concentration. Adding either lithium acetate or lithium iodide allows one to eventually obtain the same equilibrium mixture of both salts in solution.
U.S. Pat. No. 8,168,822 discloses a method of making acetic acid that includes: (a) catalytically reacting methanol or a reactive derivative thereof with carbon monoxide in the presence of a homogeneous Group VIII metal catalyst and a methyl iodide promoter in a reactor vessel in a liquid reaction mixture including acetic acid, water, methyl acetate, methyl iodide and homogeneous catalyst, the reactor vessel being operated at a reactor pressure; (b) withdrawing reaction mixture from the reaction vessel and feeding the withdrawn reaction mixture along with additional carbon monoxide to a pre-flasher/post-reactor vessel operated at a pressure below the reactor vessel pressure; (c) venting light ends in the pre-flasher vessel and concurrently consuming methyl acetate in the pre-flasher/post-reactor vessel. Iodide salt stabilizer/co-promoters may be in the form of a soluble salt of an alkali metal or alkaline earth metal or a quaternary ammonium or phosphonium salt, and in particular lithium iodide, lithium acetate, or mixtures thereof.
Under anhydrous conditions, U.S. Pat. No. 4,302,432 discloses a process for the preparation of hydrogen iodide, lithium iodide, and methyl iodide by the reaction of hydrogen and iodide in a non-alcoholic solvent using a homogeneous rhodium catalyst. Lithium iodide and/or methyl iodide are obtained by including lithium acetate and/or methyl acetate in the reaction medium.
In view of the foregoing, the need exists for an improved reaction medium to produce acetic acid with commercially acceptable yields and with low by-product formation.